It is common in the electronic art to use reference voltage in connection with complex circuits and systems. Various circuits for generating reference voltages are well known, including those which employ temperature compensation so that the reference voltage is substantially independent of the temperature over a significant range.
Bandgap reference circuits are known, for example, from:
1! Horowitz, P., Hill, W.: The art of electronics, Second Edition, Cambridge University Press, chapter 6.15: Bandgap (V.sub.BE) reference, pages 335-341; PA0 2! Ahuja, B. et. al.: A programmable CMOS Dual Channel Interface Processor for Telecommunications Applications, IEEE Journal of Solid State Circuits, vol. SC-19, no. 6, December 1984; PA0 3! Song, B. S., Gray, P. R.: A Precision Curvature-Compensated CMOS Bandgap Reference, IEEE Journal of Solid-State Circuits, vol. SC-18, No. 6, December 1983, pages 634-643; PA0 4! U.S. Pat. No. 4,375,595 to Ulmer et. al.; and PA0 5! Ruszynak, A.: CMOS Bandgap Circuit, Motorola Technical Developments, volume 30, March 1997, published by Motorola Inc., Schaumburg, Ill. 60196, pages 101-103.
The principle used in the circuits described in above mentioned references 1! and 2!, as with many other similar circuits, is based on adding two voltages whose temperature coefficients have opposite signs. One voltage is generated by a current of a given amount flowing through a diode or bipolar transistor resulting in a negative temperature coefficient and the other voltage is obtained across a resistor and has a positive temperature coefficient.
FIG. 1 is a simplified circuit diagram of reference circuit 100 known in the art. Circuit 100 receives a supply voltage between lines 101 and 102. Circuit 100 comprises resistors R.sub.a, and R.sub.b, operational amplifier OA, bipolar transistors Q.sub.1 and Q.sub.2, and current sources I.sub.1 and I.sub.2, coupled, for example, as illustrated in FIG. 1. A variety of publications, such as e.g., above mentioned references 1!, 2!, or 4!, explain how circuit 100 provides substantially temperature independent voltage V.sub.out at line 110. Arrow 105 pointing to resistors R.sub.a and R.sub.b symbolizes spikes or other noise penetrating into circuit 100 via, e.g., a silicon substrate. Such spikes occur especially in integrated circuits which have analog portions (e.g., circuit 100) in the vicinity of digital portions. The sensitivity to accept spikes increases with the geometrical size of resistors R.sub.a and R.sub.b. Also, spikes can be rectified by transistors Q.sub.1 and Q.sub.2 or by other, including parasitic components with pn-junctions.
The spikes are not the only problem. The trend in modern integrated circuits goes to small supply voltages, such as 0.8-0.9 volts or even less. Output voltages of e.g., 1.1 to 1.2 volts are generated by switched capacitors, which are very sensitive to spikes.
In prior art circuits, such as in circuit 100, currents I.sub.1, I.sub.2 flow through transistors Q.sub.1 and Q.sub.2 and through resistors R.sub.a and R.sub.b, thus loading the transistors Q.sub.1 and Q.sub.2. Resistors R.sub.a and R.sub.b should have large resistance values (in e.g., megaohms) to provide necessary voltage drops. Also, they should have enough chip area to carry currents I.sub.1 and I.sub.2. However, chip area is expensive and causes parasitic capacities making the circuit more sensitive to the above-mentioned spikes.
Accordingly, there is on ongoing need to have reference circuits which overcome these and other deficiencies well known in the art.